One critical aspect of communication systems operating in frequency division duplex FDD mode is for the terminals to provide the transmitter with reliable channel state information CSI, which allows scheduling of users in the downlink, selection of adaptive modulation and coding schemes as well as pre-processing of the data signals according to the channel conditions.
This control information is fundamental, e.g. in transmission schemes using antenna arrays at either or both transmission ends. In fact, in order to exploit the MIMO gains in terms of higher throughput and/or higher reliability of the transmit data, the transmitter should be able to form beams that match the propagation channel of the target user and possibly minimise interference from other unwanted beams. This is achieved by applying pre-coding techniques at the transmitter side, which requires accurate knowledge of the channel propagation coefficients from each pair of transmitting and receiving antenna elements.
This CSI is typically communicated to the transmitter in an FDD system by means of control information fed back periodically by the receiving terminals. The control signalling generally contains an encoded representation of a vector of channel measurements, plus a channel quality indicator CQI indicating the signal-to-noise plus interference ratio SINR at which the receiver is expecting to operate.
One common way of encoding the channel vector is by providing a codebook of vectors, which is known to both the transmitter and the terminal, such that the terminal feeds back an index corresponding to the codebook vector that is closest to the channel vector by some metric. This is basically a vector quantisation operation. This quantisation index report is carried out periodically in time and frequency, meaning that an index is fed back every given time-frequency resource block. These feedback reports show some level of correlation, which increases as the channel variations in time and frequency become slower.